Conventionally, a color laser printer has a laser beam scanning device formed of a plurality of image forming optical systems in which a plurality of light beams each using a laser diode as a light source scan a photosensitive drum. Latent images obtained on the photosensitive drum by scanning of the light beam scanning device are made visible. The visible images are superimposed and transferred on a transfer sheet to form a multi-color image. The multi-color image is fixed to form a color image.
Internal temperature of the printer varies with location, operating time or the like due to heat generation of a fixing unit and heat caused by rotational driving of a rotational polygon mirror. Since variation in magnification error due to temperature variation becomes larger, a shift occurs in the magnification error of each of the plurality of image forming optical systems. To resolve the problem, according to Japanese Unexamined Patent Application Publication No. 10-307269, a two-point synchronizing method of operating frequency modulation of a laser diode (LD) and correcting shift by providing a synchronization detecting means at each of a scanning start end and a scanning termination end of the scanning laser beam and detecting a shift amount of the magnification error due to temperature variation by these synchronization detecting means is adopted.
However, position between the synchronization detecting means provided at the scanning start end and the scanning termination end of the laser beam also varies due to variation in the internal temperature of the image forming apparatus. For this reason, only by using the above-mentioned magnification error shift correction according to the two-point synchronizing method, shift of an image on the photosensitive drum cannot be appropriately corrected. Therefore, according to Japanese Unexamined Patent Application Publication No. 2002-372676, a temperature sensor is separately provided between two synchronization detecting means to correct shift of image length on the photosensitive drum on the basis of a signal from the temperature sensor.
However, according to Japanese Unexamined Patent Application Publication No. 2002-372676, it is necessary to separately provide a temperature sensor. For example, when the temperature sensor operates abnormally, resulting in wrong detection, color shift cannot be corrected with high accuracy.